The present invention relates to a workpiece holder for a machine tool, more particularly a circular grinding machine or a lathe, of the type where the workpiece is rotated between two stationary centers. The workpiece holder is removeably attached to a driver which rotates the workpiece holder and with it the workpiece. The workpiece holder is adapted to hold the workpiece by means of elastic force, and drives the workpiece for rotation when the workpiece holder is driven by the driver. The workpiece holder comprises two adjacent, symmetrically disposed annular spring elements whose junction is provided with a throughgoing opening which forms two prismatic workpiece clamps.
A workpiece holder of this kind is known from Swiss Patent No. 663,374, according to which introduction of the workpiece in the workpiece holder and its release from the workpiece holder are effected through pressure applied to the spring elements to drive the prismatic workpiece clamps apart. An actuation by external pressure leads to a relatively large external dimensions of the workpiece holder device and furthermore, the release and clamping movements are not precisely defined. This leads to a restriction of the field of application, in particular in the direction of small work pieces.
Another workpiece holder is known from European Patent Application No. 782,898, according to which the clamping chuck of a lathe acts upon radially moving clamping springs which actuate centering points in order to chuck the workpiece. The same drawbacks as explained above apply here.
The manufacture of small high-precision parts, e.g. injection needles for the automobile industry, makes high demands on the workpiece holder while the workpiece is held between two stationary centers. Due to the small dimensions of such small precision parts, on one hand, and their low inherent flexural stability, on the other hand, the precision geometrical requirements and the process safety of the grinding operation cannot be ensured by conventional workpiece holders such as heart shaped holders, driving centers and the like.
The essential drawbacks of the conventional workpiece holders of the prior art are
deformations of the workpiece by the clamping force of the workpiece holder acting upon the workpiece;
a large rotating mass of the workpiece holder with respect to the stability of the workpiece;
the force transmission from the machine to the workpiece holder produces an excessive transversal force with respect to the stability of the workpiece, and the geometrical dimensions limit the workable length of the workpiece.
In the future, the developments in fuel injection technology will lead to even smaller workpiece dimensions and higher precision requirements, and thus to an even greater significance of the workpiece holder.
Although according to the first cited document, the mass of the workpiece holder is reduced in view of the then known prior art, it becomes increasingly difficult to reduce the size of the chucking of the parts.
It is an object of the present invention to provide a workpiece holder for machine tools which eliminates the above-described drawbacks while consistently separating the finctions of clamping the workpiece and that of transmitting the torque of the machine to the workpiece, eliminating as much as possible the lateral forces, and which offers a small rotating mass of the workpiece holder and a safe handling even of small parts. This object is attained by a workpiece holder which comprises respective longitudinal openings which are disposed along the axis of the points of said prismatic workpiece clamps and provided with prismatic ends for receiving respective, correspondingly shaped bolts of the drive of the circular grinding machine in order to spread apart the spring elements.
Further advantages and developments of the invention are defined in the dependent claims.